Not eating for 3 days won't cause you to drop or lose much weight for just those 3 days even though fasting for at least 1 day would help cleanse your body and help you on the path to losing weight.
It can take at least a whole month to even safely lose 10 lbs of weight so you might drop 1/2 a lb in a 3 day period of not eating.
But if you go 3 days without eating anything at all then you will likely just make yourself sick and you'll become weak because our bodies need energy to thrive and when you don't eat anything for more than a day then you'll start getting weaker and could pass out.
I would not recommend going without food for 3 days but 1 day every other day could help you lose weight.
You can safely lose between 40 lbs to 80 lbs of weight in a year so do it gradually and exercise along with the fasting and eat healthy and you should see a good amount of weight loss after 6 months to a year.
Don't do anything that can harm your health when trying to lose weight.
You could at least eat a bowl of fruit or a bowl of oatmeal or at least eat a couple pieces of bread to give you at least a bit of energy in those other 2 days when fasting.